This research will define the cellular basis for differential or selective inhibition of neoplastic cell growth by the protein kinase antagonists flavopiridol and UCN-01, with special focus on cell cycle arrest and apoptosis; determine the biochemical basis for, and consequences of, flavopiridol's and UCN-01's respective action on the cyclin dependent kinase(CDK) family of cell cycle regulatory molecules; and define pharmacodynamic endpoints in vitro and in vivo for use in clinical trials with flavopiridol and UCN-01. In the past year, this project has examined the capacity of a series of flavopiridol analogs to inhibit CDK2, for which crystals for X-ray diffraction could be generated by Dr. Kim (UC Berkeley). It was subsequently possible to diffuse deschloro flavopiridol into CDK2 crystals, and actually demonstrate the drug's presence in the ATP binding site (Proc. Natl. Acad Sci. 93, 2735, 1996). We also demonstrated that flavopiridol could potently inhibit CDK 2 as well as CDK4, and that cells arrested in G1 by flavopiridol show dephosphorylation of the retinoblastoma tumor suppressor protein (pRb), a known substrate of CDKs 2 and 4 (Cancer Res. 56, 2973, 1996). We are currently conducting a Phase I Trial with continuous infusion flavopiridol. We have demonstrated that UCN-01 can abrogate the G2 checkpoint in irradiated cells. Exposure to UCN-01 results in apparent sensitization to both radiation or cisplatin. This effect is most evident, in a MCF7/papilloma E6 transfected in cells with altered (vs wt) p53 function (J. Natl. Cancer Inst 88, 956, 1996). We have recently also opened a Phase I trial of UCN-01.